X-ray dosage indicator



Oct. 30, 1945. w. H. DIMSI DALE ET AL 2,337,887 Q X-RAY DOSAGE:ISNDICATOR Filed June 27, 1944 11v ENTO .r if" 0 r/l an; I we I.JITTORNEY Patented Oct. 30, 1945 X-RAY DOSAGE INDICATOR Wilfrid HampdenDimsdale and Alfred Eli Clarke,

Ilford, England, assignors to Ilford Limited, [1- ford, England, aBritish company Application June 27, 1944, Serial No. 542,392 In GreatBritain July 27, 1943 9 Claims.

This invention relates to radiography and particularly to devices foruse in detecting and estimating quantities of X-radiation andgammaradiation.

For some time it has been customary for workers with X-rays andgamma-rays to carry on their person a piece of photographic filmprotected against light but not against X-rays or gamma-rays. After someperiod of time, for ex-' ample a week, the film is developed in a normalphotographic developer and the density of the developed silver (if any)serves as an indication of the extent to which X-rays and gamma-rayshave affected the film. Since X-rays and gammarays are dangerous tohealth, the film thus serves as an indication of whether the worker hasbeen exposed to a dangerous quantity of radiation.

A difiiculty arises, however, in that the natural fog density of thefilm employed is not usually known. The fog density is the density ofthe silver deposit formed on development when the film has not beenexposed either to light or to X-rays or gamma-rays, and this value isliable to vary from film to film and with the age of the film. Thus,especially for small quantities of X-radiation or gamma-radiation, it isnot possible to be certain how much of the developed silver density isdue to X-radiation or gammaradiation and how much due to the natural fogdensity of the film.

According to the present invention, this difficulty is overcome byproviding, close to the lightsensitive material, e. g. photographicfilm, a mask having two or more areas of different permea-' bility toX-rays or gamma-rays, one area of the mask being substantiallyimpermeable to such rays, the said mask covering only a fraction of thearea of the film.

The mask may be in the form of a metal sheet having two or more areas ofdifferent thicknesses and therefore different permeability to'X- rays orgamma-rays, or may consist of two or more separate or adjacent sheets ofdifferent metals having differing permeability to X-rays or gamma-rays.More than one such mask may be provided, e. g. two masks each in theform of a metal sheet having two or more areas of different thicknessesand therefore different permeability to X-rays or gamma-rays, and beingformed of the same or different metals. A single mask may be provided onone side of the film, or two or more similar or different masks maybeprovided on the opposite sides of the film, or the differing areas ofa single mask may be distributed between the two sides of the film.

By employing a mask element having an area which is substantiallyimpermeable to X-rays or gamma-rays, it is possible, where the amount ofX-rays or gamma-rays which have affected the film is small, todistinguish adequately between the density of the developed silver whichis due to X-radiation or gamma-radiation and the natural fog densitywhich is that of the film covered by the substantially impermeable areaof the mask. If the mask is provided on one side of the film only, it isimportant that an indication be given on the outside of the filmcontainer showing which side of the film is to be exposed to the X-raysor gamma-rays, i. e. the side on which the mask is provided is to bethat facing the source of X-rays or gamma-rays' It is preferred,however, to provide similar masks on both sides of the film so that itbecomes immaterial which side faces the source of X-rays or gammarays,and preferably, though it is not absolutely necessary, such masks shouldbe in register with one another.

As stated above, the mask may consist essentially of sheet metal havingareas of different permeability to X-rays or gamma-rays. Then if thepermeability characteristics of the metal sheet to X-rays or gamma-raysare known it is possible by examining the density of the image on thedeveloped film to obtain an accurate estimation of the quantity ofX-rays or gamma-rays which have fallen on the film. Such a mask ofgraduated permeability may be constructed so that the permeabilityranges from a maximum at one end of the metal sheet (the end of minimumthickness) to a minimum at the other end (the end of maximum thickness)and the end of minimum permeability may be arranged to be such that anyimage formed on the area of the film covered by such end may beregarded, for practical purposes, as representing only the natural fogdensity of the film. The metal sheet may be constructed to have asmoothly varying thickness (and therefore degree of permeability) fromone end to the other, but it is preferred that the thickness should varyin steps. This may readily be effected by the suitable folding of astrip of metal foil so that in the folded strip there is an area ofsingle thickness, another of double thickness and so on as required. Theuse of mask elements so constructed forms an especially valuable andpreferred feature of the present invention.

The use of such a mask element enables an estimate to be made on thequantity of X-rays or gamma-rays which have affected the film andaffording an indication of the hardness or suite.

ness of the X-rays which have affected the film. It may arise'that asingle mask formed, for example, in lead foil, does not afford asufiiciently wide range of impermeability to the rays. In this case twomasks may be provided, one in a material which has a high degree ofimpermeability to X-rays or gamma-rays, e. g. lead or a lead alloy, andone which has a lower degree of impermeaability to such rays, e. g.copper or a copper alloy. Thus, it is found that lead foil of ure 2 andthe coincident folds CD and GH along either the line PR or the line QSof Figure 2.

In this way it is arranged that the mask is of varying permeabilityalong its length, stepwise in the ratio 4:2:1, that it is exactly inregister on the two sides of the film I, and that it cannoteasily bedislodged during normalhandling of the assembly. An alternative form ofconstructing a similar mask is illustrated in Figures 4 and 5. Themethod of folding the mask is the same as for Figure 3A, but isduplicated for the right and left sides of the original mask element.The final fold along AB brings the mask to the form shown in Figure 5.This construction provides only a single thickness of foil at AB andthus tends to reduce bulkiness of the marginal edge RS of the assembly.With this about .004" thickness folded as indicated above is especiallyuseful where the incident radiation g is, for example X-rays generatedat 100-150 kvp., whilst at lower kilovoltages a brass or copper mask ismore suitable.

The accompanying drawing illustrates suitable forms of the invention butare not to be regarded as limiting the invention in any way.

In this drawing Figure 1 represents a cross-section through an assemblyof sensitive film, mask and protective coverings, Figure 2 represents anisometric view of this assembly cut away to show the individual layers,Figures 3A, 3B, 3C and 3D illustrate one method of constructing a maskelement according to the invention, and Figures 4-8 inclusive illustratealternative forms of mask.

Referring to Figures 1 and 2, the assembly consists of: a sheet ofsensitive film l or paper, which may be coated with X-ray sensitiveemulsion on one or both sides; two sheets of black paper 2 provided oneither side of the film l to protect the latter from exposure to light,a mask element 3 which is arranged to fold along one edge of theassembly, a protective sheet 4 and a wrapper 5 which is folded round theassembly and overlaps the margins of the sheet 4, and a cover sheet 6.The wrapper 5 is stuck, at its marginal edges, to the sheet 4 and thecover sheet 6 is stuck, at its marginal edges, to the marginal edges ofthe sheet 5. If desired the cover sheet 6 may also be locally stuckdirectly to the sheet 4'. By arranging that the adhesives used and thewrapping papers are water-resistant and that the wrapper 5 and sheet 4are impervious to light, a waterand lightresistant assembly is produced.

Figures 3A, 3B, 3C and 3D illustrate a method of forming the maskelement 3 of Figures 1 and p 2. A sheet of metal foil, e. g. lead foil.004 inch thick, as illustrated in plan in Figure 3A and in section inFigure 3B is employed. First the foil is folded along the line EF ofFigure 3A thus bringing it to the form illustrated in Figure 3C. Theline CD of Figure 3A then lies over the line GH of Figure 3A. Next thefoil is folded along the coincident lines CD and GH, which brings thefoil to the form shown in Figure 3D. It will be seen from the drawingthat the result of these folds is to make the thickness of one end ofthe sheet equal to four thicknesses of foil,the middle portion of thesheet equal to two thicknesses of the foil and the other end of thesheet a single thickness of foil. Finally the foil is folded along theline AB (Figure 3A) and in this condition can be used as the mask forthe assembly of Figures 1 and 2 by arranging that the fold AB lies alongthe marginal edgeRS of Fig.,

construction the steps of the mask are of increasing thickness in thedirection R to P instead of, as in the mask of Figure 3A, of increasingthickness from R to S or S to R.

The construction of the assembly can conveniently be of the general sizeanddimensions of a dental radiographic film pack of conventional design,and the cover sheet 6 of Figures 1 and 2 can conveniently consist of alabel carrying a printed description of the. product. If desired, thiscover sheet 6 and the protective sheet 4 may be provided with a cut sothat on bending the assembly with the cut portion on the convexside atab is formed which may be pulled to tear away the wrappings from thefilm. A device of this character is described in British patentspecification No. 423,809 in connection with a dental radiographic filmpack assembly.

It will be appreciated that there is considerable scope for variation ofthe precise structure of the assembly and of the mask or masks employed,and that the particular form of the invention illustrated in theaccompanying drawing represents only one of numerous variations. Thus,for example, the mask may consist of several strips of different metalsof uniform thickness; a mask may be provided on one side only of thefilm; where masks on both sides are employed they need not be inregister nor need they be formed of one piece folded round the edge ofthe assembly as shownin the drawing. Moreover, instead of a mask havinga thickness ratio varying as 4:2:1 as in Figure 3, other methods offolding the metal foil may be used to give other ratios, for examplethose illustrated in section in Figure 6, where the ratio is 1:3:4, inFigure 7 where the'ratio is 1:3:5, or in Figure 8 where the ratio is1:3:2.

Instead of comprising a single film element, the assembly may comprisetwo such elements separated by a layer of metal foil which issubstantially impermeable to X-rays and gammarays. In this'way the twofilms may be used separately to record the quantity of X-rays orgamma-rays present under different conditions, or over different periodsof time, merely by changing the side of the assembly facing the sourceof X-rays or gamma-rays.

If desired, in order to secure the mask element more firmly in position,it may be stuck to one of the paper sheets in the assembly. For example,an extra sheet of paper may beintroduced between the lower sheet of.paper. 2 and the lower part of the mask 3 in Figure 1, this extra sheetof paper being coated with an'adhesive on its lower side and the lowerpart of the mask 3 being stuck to it. Such an arrangement, by securingthe mask in position initially,

sometimes makes for simplicity in building up the assembly.

Although in the assembly illustrated in the accompanying drawing anddescribed above the mask or masks are included within the package whichcontains the film, the masks may be included in a separate permanentcassette into which a packet of film may be inserted. Thus, for example,a cassette made of card board or similar material may include a mask ormasks in accordance with this invention and be adapted to hold a dentalradiographic pack of conventional design so that the mask or masks lieon the outside of the pack and thus exert their masking effect on thefilm within the pack.

What we claim is:

1. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gamma-rays, and provided with a mask consisting of a metalsheet having at least two areas of different thicknesses, the thickestarea of the mask being substantially impermeable to such rays, the saidmask covering only a fraction of the area of the film.

2. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gammarays, and provided with a mask consisting of a metalsheet folded to provide areas which are different multiples of thethickness of the sheet, and thus areas of different effective thickness,the thickest area being substantially impermeable to X-rays andgamma-rays, the said mask covering only a fraction of the area of thefilm.

3. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gammarays, and provided on opposite sides of the film withmasks each consisting of a metal sheet folded to provide areas which aredifierent multiples of the thickness of the sheet, and thus areas ofdifferent efiective thickness, the thickest area of at least one suchmask being substantially impermeable to X-rays and gamma-rays, the saidmasks each covering only a fraction of the area of the film.

4. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gammarays, and provided on opposite sides of the film withmasks each consisting of a metal sheet folded to provide areas which aredifferent multiples of the thickness of the sheet, and thus areas ofdiiierent effective thickness, the thickest area of at least one suchmask being substantially impermeable to X-rays and gamma-rays, the saidmasks each covering only a fraction of the area of the film, the saidmasks being in register with one another.

5. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gamma-rays, and provided on the same side of the film withtwo masks each consisting of a metal sheet folded to provide areas whichare different multiples of the thickness of the sheet, and thus areas ofdifferent efiective thickness, the thickest area of one such mask beingsubstantially impermeable to X-rays and gamma-rays, the said masks eachcovering only a fraction of the area of the film, one of such masksbeing formed of a metal highly impermeable to X-rays and gammarays andthe other being formed of a metal of relatively low impermeability toX-rays and gamma-rays.

6. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gammarays, and provided with a mask consisting of metal foilfolded to provide areas which are different multiples of the thicknessof the foil, the said foil being further folded symmetrically along oneedge of the film to provide similar masking areas of difierent effectivethickness, in register, on the two sides of the film, the thickest areaof the foil being substantially impermeable to X-rays and gamma-rays andthe said mask covering only a fraction of the area of each side of thefilm.

7. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gammarays, and provided with a mask consisting of a leadsheet having at least two areas of different thicknesses, th thickestarea of the mask being substantially impermeable to such rays, the saidmask covering only a fraction of the area of the film.

8. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gammarays, and provided with a mask consisting of a leadsheet folded to provide areas which are different multiples of thethickness of the sheet,

and thus areas of diiferent effective thickness.

the thickest area being substantially impermeable to X-rays andgamma-rays, the said mask covering only a fraction of the area of thefilm.

9. A film pack for use in detecting and estimating quantities ofX-radiation and gammaradiation which comprises a sheet of photographicfilm contained in a package protecting it against light but not againstX-rays and gammarays, and provided on the same side of the film with twomasks each consisting of a metal sheet folded to provide areas which aredifferent multiples of the thickness of the sheet, and thus areas ofdifferent effective thickness, the thickest area of one such mask beingsubstantially impermeable to X-rays and gamma-rays, the said masks eachcovering only a fraction of the area of the film, one of such masksbeing formed of lead and another copper.

WILFRID HAIVH DEN DIMSDALE. ALFRED ELI CLARKE.

